Glass fibers and glass fiber strands have been used before in the art to produce various types of glass fiber mats for use as reinforcement material. The basic principles of mat-making are well known in the art and are fully described in the book entitled "The Manufacturing Technology of Continuous Glass Fibers" by K. L. Lowenstein, published by the Elsevier Publishing Company, 1973 at pages 234 to 251. Typical processes for making mats of continuous fiber glass strands are also described in U.S. Pat. Nos. 3,883,333 (Ackley) and 4,158,557 (Drummond).
Generally, mats formed by these processes are needled in order to improve their mechanical integrity. The needling operation typically used is described more fully in U.S. Pat. Nos. 3,713,962 (Ackley), 4,277,531 (Picone) and 4,404,717 (Neubauer, et al.) Mechanical integrity can also be imparted to mats by depositing a resin on its surface and then curing or melting it so that individual strands are bonded together.
A particular utility for glass fiber mats is in the reinforcement of resinous or polymeric materials. The presence of an integrally molded glass fiber mat increases the strength of otherwise unreinforced material. Usually, the mat and a molten resin are processed together to form a thermosetting or thermoplastic laminate. Thermoplastic laminates are particularly attractive for use in the aircraft, marine, and automotive industries since they may be reheated into a semi-molten state and then stamped into panels of various shapes such as doors, fenders, bumpers, and the like. It is important that glass mats used to make laminates have as uniform a fiber density distribution as possible. If a non-uniform density mat is used for reinforcement purposes, the reinforced products produced therefrom may have a substantial variation in strength since some areas will be weaker due to the lack of glass fiber reinforcement while others will be stronger. Even more important is the need to insure that the glass fiber mat flows or moves freely within a thermoplastic laminate during the stamping operation in order to provide uniform strength properties to the final component.
In the production of continuous strand mats by the aforementioned patented processes, a plurality of strand feeders are positioned above a moving belt or conveyor, typically a continuously driven, flexible, stainless steel chain or cable. The strand feeders are reciprocated or traversed back and forth above the conveyor parallel to one another and in a direction generally perpendicular to the direction of motion of the moving conveyor. Strands composed of multiple glass fiber filaments are fed to the feeders from a suitable supply source such as a plurality of previously made forming packages. Each feeder apparatus provides the pulling force necessary to advance the strand from the supply source and eventually deposit it upon the surface of the moving conveyor. In a typical production environment, as many as 12 to 16 such strand feeders have been used simultaneously with one another so as to produce a glass fiber mat.
It is also well known in the art that the feeder can act as an attenuator to attenuate glass fibers directly from a glass fiber-forming bushing and eventually deposits strand formed therefrom directly onto the conveyor as described by Lowenstein, supra at pages 248 to 251 and further illustrated in U.S. Pat. Nos. 3,883,333 (Ackley) and 4,158,557 (Drummond).
In the operation of the traversing system described above, the configuration of the equipment used suffers from inherent limitations on its mechanical durability. First, the feeders are quite heavy, usually weighing between 30 and 50 pounds or more. When this heavy apparatus is reciprocated across the width of the conveyor, the traverse speed is limited due to the momentum of the moving feeder and the impact forces which must somehow be overcome or absorbed upon each reversal of direction. This mechanical limitation on the traverse speed also limits the rate of mat production. Secondly, the constant reciprocating motion of the feeders causes vibration to occur and this can result in a great deal of wear on the feeder mechanisms and their track guides, which may eventually lead to mechanical failure.
In U.S. Pat. 3,915,681 (Ackley), a reduction in the vibration normally associated with the reversal of a feeder was accomplished by the use of a traversing system in which a feeder was advanced by a continuous chain driven by a motor. The chain had affixed to it an extended member, or pin, which engaged a slot milled into the feeder carriage. The slot was positioned so that its length was parallel to the direction of motion of the chain. The length of the slot was substantially greater than the diameter of the pin. Thus, as the feeder traveled in one direction, the pin exerted the force necessary to advance it by pressing against the periphery of the slot. When the feeder reversed its direction, the pin first slid along the length of the slot until it contacted the opposite periphery at which point the motion of the feeder was reversed. At the termination point of the reciprocation stroke, the feeder contacted a shock absorber which decelerated it and absorbed the impact due to the change in momentum. Later, as an improvement on this design, the shock absorbers were replaced with gas pistons and a reservoir capable of storing the absorbed energy was used to help accelerate the feeder in the opposite direction (See U.S. Pat. No. 4,340,406 (Neubauer, et al.)).
A second problem with the systems taught by the prior art was the ability to produce a consistent mat of uniform strand density. In the deceleration/acceleration cycle of the feeders, more glass strands tended to accumulate on the conveyor at the terminal end of each traverse stroke. This resulted in a mat tending to be thicker near its edges than in the more central portions thereof. The buildup of additional glass strands near the edges of the mat was caused when the feeder reversed its direction since the feeder was locally resident for a greater duration of time over those portions of the mat where the deceleration/acceleration cycle occurred, i.e., the edges. As long as the feeder was paying out strand at a constant rate during the turnaround cycle, the edges of the mat could do nothing but accumulate a greater depth of glass strand than was present in the more central regions. Thus, in order to produce a finished mat having a uniform density, it was necessary to trim the mat as it left the conveyor. This reduced the efficiency of the process by a substantial amount since the trimmed material was disposed of as waste.
Thus, despite the advances made by the prior art, there still exists a need to (1) rapidly reverse the feeder apparatus during its turnaround cycle, (2) minimize the mechanical vibration associated with a rapid turnaround of the feeder apparatus, and (3) control mat edge uniformity and density. As will now become evident from the remainder of the disclosure, an improved mat-making method is provided which satisfies these needs.